The present invention relates to wireless transmission, and more particularly to a method for estimating a carrier frequency offset for an interleaved OFDMA uplink receiver.
As a widely used technique for high data rate wireless transmission, the OFDM (Orthogonal Frequency Division Multiplexing) technique makes use of a set of overlapping but orthogonal sub-carriers to reach high spectrum efficiency. Inheriting from OFDM, OFDMA (orthogonal frequency division multiple access) has been proposed in many broadband wireless systems to provide more flexible wireless access scheme and to take more advantage of diversity gain by allocating a user a set of permutation-driven interleaved sub-carriers that guarantee a large sub-carrier spacing for each user.
Working in a mobile wireless environment, OFDMA is subject to synchronization errors, such as the misalignments from the terminals to the base-station, the discordances between the oscillator of the base-station and those of the terminals, and Doppler shifts of the terminals. Like the OFDM technique, OFDMA is so sensitive to the synchronization errors that a small frequency offset would lead to the loss of the orthogonality among the sub-carriers, and that a short time delay would result in the complex exponential twiddle on the frequency-domain.
The time-domain received signal in uplink is a multiplex of the multi-user signals that are subject to the different frequency offsets, time delays, and channel distortions. The interleaving topology of OFDMA deteriorates this issue by turning the ICI (inter-channel-interference) among the sub-carriers to the MAI (multiple access interference) among users. Besides, synchronization errors start to fluctuate when a user moves fast.
In order to keep the synchronization of terminals and base-station, a ranging process is taken to detect the synchronization errors of a terminal and to control the adjustment of the terminal's transmission in a close loop between this terminal and the base-station.
Functionally, the ranging process is classified into initial ranging and periodic ranging. Initial ranging takes place when a terminal is (re-)registered into the network; while periodic ranging is performed to keep the synchronization between a terminal and the base-station during its constant transmission. Usually, the initial ranging consumes more signaling resources by transmitting multiple OFDM symbols training sequence in uplink by which the base-station receiver is able to estimate the synchronization errors accurately but in relative long time interval; and the periodic ranging needs single OFDM symbol training sequence in uplink by which the base-station receiver can estimate the synchronization errors in a short time interval.
The synchronization errors of a low mobile or even fixed terminal may change so slowly that after the initial ranging reduces the synchronization errors of the terminal under an acceptable criterion, the base-station hardly performs any periodic ranging for its maintenance. But, once the terminal speeds up, its synchronization errors may fluctuate dramatically so as to require frequent periodic ranging processes. Among the synchronization errors, frequency offset is the most important one, for it would destroy the orthogonality causing MAI. (The phase rotations resulting from the time delays can be, more or less, compensated by the channel estimator.)
In conventional OFDM system uplink, a common pre-defined training sequence (ranging code) is transmitted on the overall sub-carrier in one OFDM symbol. And with a repetitious structure on the time-domain, this training sequence can be taken by ML (maximum likelihood) algorithm to estimate the frequency offset. However, this kind of ranging code doesn't work in OFDMA uplink, because 1) the ranging code by no means occupies the overall band; and 2) it isn't a common pre-defined training sequence but a CDMA (code division multiplex access) code generated by PN (pseudo-noise) polynomial in order to distinct terminals.
An alternative to estimate the frequency offset in OFDMA uplink is to repeatedly transmit a CDMA ranging code in multiple consecutive OFDM symbols (on the partial band) with phase continuity on the time-domain. Then, the base-station receiver can still apply ML to the repetitious training sequence. This method is taken in the initial ranging.
IEEE802.16e OFDMA system adopts initial ranging and periodic ranging. FIG. 1 shows the mandatory initial ranging and periodic ranging.
FIG. 1(a) is the time-domain illusion 100 of the initial-ranging transmission. The initial-ranging transmission is performed during two consecutive OFDM symbol periods 102 and 104 with copies of specific duration of last samples as CP 106.
FIG. 1(b) is the time-domain illusion 110 of the periodic ranging transmission. The periodic ranging transmission is performed during one OFDM symbol period 112 with a copy of specific duration of last samples as CP.
These repetitions of symbol period, termed CP, provide multipath immunity as well as tolerance for symbol time synchronization errors.
Initial ranging serves registering a new terminal into network. The time delays, frequency offsets, and transmission power of an un-registered terminal shall be estimated and adjusted to guarantee its on-going reliable transmission. A base-station grants an initial ranging opportunity by allocating ranging channels in an uplink sub-frame. The grant information is encapsulated into a UL_MAP that is broadcast in the downlink sub-frame. Given a ranging opportunity, terminals collide on these ranging channels by transmitting a CDMA code, denoted as an initial ranging code, which is randomly selected from a CDMA code candidate set specified by the base-station. This ranging code will be detected and transmitted together with the parameter adjustment message in a ranging response during the next downlink opportunity to notify the terminal to be adjusted.
Periodic ranging serves re-synchronizing a terminal with the base-station. A base-station grants a periodic ranging opportunity in an uplink sub-frame. The terminals collide on the ranging channel by transmitting a CDMA code that is randomly selected from a candidate set specified by the base-station.
A prior art solution has been proposed by Young-Ha Lee et al. This solution is applied to solve synchronization of an uplink between a subscriber station and a base-station by utilizing the ranging system in a multiple access wireless communication system of OFDMA.
However, this solution is only restricted to timing synchronization rather than frequency synchronization. Since OFDMA system is very sensitive to frequency synchronization errors in a mobile environment, performing the synchronization process without considering frequency offset becomes inapplicable in practice.
Another prior art solution has been proposed by Chang-Wahn Yu et al. This solution is applied to process the ranging channels to measure the propagation delay and the power of each subscriber station.
In an OFDMA system, each subscriber station has different carrier frequency offsets if the system is not synchronized. The orthogonality among these subcarriers of the different subscriber stations are thus destroyed due to MAI. Therefore, the insufficiencies of the above solution are that this solution does not take into account of frequency offset either.
A challenge to the periodic ranging is how to estimate the CFO based on a single-OFDM-symbol CDMA ranging code, when it comes to a mobile environment.